Chapter 18: Genetics of
Cancer
and Cell-Cycle Regulation
What is Cancer?
• Large number of complex diseases
• Behave differently depending upon cell type
from which originate
– Age on onset, invasiveness, response to treatment
• Common general properties
– Abnormal cell growth/division (cell proliferation)
• If only this is a benign tumor
• When grow in culture without contact inhibition are
referred to as transformed
– Spread to other regions of body (metastasis)
• Malignant tumors
Cancer
• Second leading cause of disease in Western
Countries
• 1 million new cases per year in U.S.
– 500,000 per year die
• War “declared” on cancer approximately 30
years ago
• Slowly treatments are changing/improving
based upon better genetic understanding of the
varieties
Cancer Rates in US
Age and Cancer
• Note log scale for
incidence rate
Abnormal Cell Growth
Cancer is a Genetic Disease
• Genome alterations
– One nucleotide to large-scale chromosome
rearrangements, amplifications and deletions
– Mostly in somatic cells (unless associated with
inherited risk—about 1% of total)
– Alter cellular functions
• DNA repair, cell division , apoptosis, cellular
differentiation and cell-cell contact/communication
Molecular Biology of Neoplasia
Retroviral oncogenes:
Abl, akt, erbB, ets, fos, kit, myb, myc, raf, ras, rel,
src, yes
Proto-oncogene: normal gene that can undergo a
genetic change to become cancerous.
Oncogene: a gene that causes a normal gene to
become cancerous. Mutant overactive form of a protooncogene.
Tumor suppressor gene: a recessive mutation in an
inhibitory gene, thus inactivating the gene. Loss of
function causes tumorigenesis.
Functions of Cancer Causing
Genes/Alleles
• Many disrupt control of cell cycle
• Oncogenes
– Proto-oncogenes
• Normal genes that if mutated may act to make a cell cancerous
• Recessive, cancer causing forms active and stimulates cell division
• C-oncogenes and v-oncogenes
• Tumor suppressors
– Genes whose products act to regulate cell cycle
– Loss of gene product function contributes to cancer process
– Recessive, commonly involved with inherited risk
• About 200 proto-oncogenes and tumor suppressor genes
Oncogenes are identified through their
dominant transforming effects
Changes observed when a normal tissue
culture cell is transformed by a tumor virus
or an expression vector carrying the
oncogene
1. Alterations in the nucleus
2. Plasmamembrane related abnormalities
3. Adherence abnormalities
4. Growth and division abnormalities
5. Defective differentiation
6. Inability to undergo apoptosis following
DNA damage
Normal and Cancer Karyotypes
• Chromosome painting
(a) is a normal cell
(b) is a “very messed up” cancer cell
Clonal Origin of Tumors
• Tumor arises from a single cell
• Burkitt’s lymphoma
– Translocation involving chromosome 8 (myc) and
either chromosomes 2, 14, or 22 (near an
immunoglobulin gene
– All cells from a patient have breakpoints at exactly
the same points as shown by DNA sequence analysis
– Cancer cells in tumors of females all use same X
chromosome (same one in Barr body)
Multistep Process
• Cancer requires mutation of multiple genes
• Age relationship with cancer consistent with this
– If one mutation caused cancer then rate would be
constant independent of age
• It increases dramatically with age…
• Delay between carcinogen exposure and onset
– 5-8 year delay between carcinogen exposure
(Hiroshima and Nagasaki) and onset of leukemia
– 15 year delay between tuberculosis X-ray treatment
and onset of breast cancer
Multistep Process…Continued
• Cancers often develop in progressive steps
– From mildly aberrant cells to malignant
– See figure 18-3
– Process called tumorigenesis
Tumorigenesis
of Cervical
Cancer
Properties of Cancer Cells
• Genetic instability
– Mutator phenotype
– Duplicating, losing and translocating chromosomes
or portions of them common
• Chronic myelogenous leukemia (CML)
– Chromosome 9/chromosome 22 translocation
– BCR gene fused to ABL (protein kinase)
– Mutant signal transduction protein stimulates cells
constantly to proliferate
Genome Instability
• Double minutes (DMs)
– Miniature
chromosomes giving
many copies of rgion
• Homogeneous staining
regions (HSRs)
– Tandem gene
duplications
Chromosomal Translocation in
CML
Xeroderma Pigmentosum
• Failure to remove pyrimidine dimers from
DNA
– Excision repair defect
• Patients often develop skin cancer and must
stay out of sunlight
HFNPCC
• Hereditary nonpolyposis colorectal cancer
• Higher than normal rates of colon (first
noted) but also elevated rates of ovary,
uterine and kidney cancers
• 1/200 persons, autosomal dominant
• Eight genes associated and four involve
mismatch repair systems
HNPCC Pedigree
• Colon, Stomach endometiral, pancreatic, bladder
• Orange also other cancers, multiple slashes unknown cause
of death
Role of cell division in tumor progression
Tumors arise from cells with
DNA damage or mutant DNA
that divide uncontrollably.
Cancer cells lose normal
restraints for replication of
damaged DNA and G1/S
progression of cells with
damaged DNA.
Increased probability of tumor
progression by further genetic
change.
Proto-oncogenes can be converted into oncogenes
Proteins that control cell
growth (proto-oncogenes
and cell cycle checkpoints)
1. Growth factors and receptors:
(EGF/EGFR, IGF/IGFR,
PDGF/PDGFR)
2. Intracellular transducers:
GTP binding proteins: Ras
Protein kinases: Src, Raf
3. Intracellular receptors: ER, RAR
4. DNA repair proteins: BRCA1
5. Cell cycle control proteins:
cyclins, cdkis, Rb, p53
Proto-oncogene may become
converted to an oncogene by a
mutation that results in
increased intrinsic activity of the
protein product
6. Transcription factors: myc, jun.
fos, myb
7. Anti apoptotic proteins: Akt,
Bcl-2
Defects in Cell Cycle
Regulation
• Cell cycle
• G1, S, G2, M phases
• Progression through cycle is regulated and
specific blocks or checkpoints exist
• Nondividing cell (quiescent) is in an
extended G1 phase called G0
– Cancer cells never enter G0
Cell Cycle
Cell Cycle Checkpoints
• G1/S
– Monitors cell size and for DNA damage
• G2/M
– Replication complete, DNA damage?
• M
– Spindle fibers connected, etc.?
• G0
– Does body require more of my type of cell?
Regulators of Cell Cycle
• Cyclins and cyclin-dependent kinases (CDKs)
• Cyclins synthesized and destroyed in a precise
pattern
– A cyclin bind to a specific CDKs, activating it
• Other proteins phosphorylated/activated
• CDK4/cyclinD activate transcription factors for genes
such as DNA polymerase delta and DNA ligase
• CDK1/cyclinB trigger events of early mitosis
(chromosome condensation, nuclear membrane
breakdown, etc.)
Cell cycle regulatory genes can be
oncogenes or tumor suppressors
Cyclins as oncogenes
2. Oncogenes that encode growth factors or their
receptors
Translocation of Tel transcription factor onto PDGF causes dimerization (activation) in the absence of
correct ligand. Found in leukemias
• mutant receptor – always active even without binding
ligand
•mutant Gsα – always active – always signaling to
adenylate cyclase
Steroid hormone receptors as oncogenes:
ER, AR, myl (RAR)
Steroid hormone receptors can act as nuclear
transcription factors and activate transcription of
other oncogenes such as cyclins, myc, myb
Cyclin Levels
Activation
of CDKs
Tumor Suppressor Genes
Mutations in tumor suppressor genes are usually
recessive in their effects on the individual cell.
1. Cell Cycle Regulators:
• Cdkis (cyclin-depndent kinase inhibitors)
• Rb (Retinoblastoma protein)
• P53
2. Signaling proteins: APC (adenomatous polyposis coli)
3. DNA damage repair proteins: BRCA1 (Breast cancer
gene)
4. Enzymes: PTEN (phosphatase and tensin
homolog deleted in chromosome ten)
Cell cycle checkpoint proteins regulate
G1/S transition
A. CDK inhibitors
activated by:
• P53 (p21 = cip1)
• Vitamin D (p21)
• Adhesion (p27)
• TGFb (p15, p27)
B. Rb protein family:
pRb, p107, p130
p15 gene
Induction of p15 results in inhibition of Cdk4,6/cyclin
D complexes and cell cycle arrest at G1.
P15 and Smads are tumor suppressors
Oncogenes/Proto-oncogenes
• Cyclin D1 and Cyclin E are proto-oncogenes
– Often amplified or over expressed due to other
mutations (e.g. translocation) in many cancers
• cyclinD1 allows for DNA replication (S phase)
• Over expression seems to contribute to cell’s
progression from G0 phase and begin division
Oncogenes encoding signaling molecules: ras
ERb-B is an oncogene in glioblastomas, renal cell, salivary gland, squamous
cell, breast, gastric, ovarian, and esophageal carcinomas
Ras is an oncogene in bladder, breast, lung, and head and neck carcinomas
Ras is activated by a point mutation in the
GTP binding domain
Protooncogene
First human activated protooncogene
Homolog of Harvey-ras and Kirsten-ras from rat
sarcoma
ras Proto-oncogenes
• Involved in signal transduction pathway
– As are many proto-oncogene products
• ras family genes mutated in 40% of all cancers
• Involved in signal transduction pathway from
growth factor receptor to nucleus
– G protein
– Mutant form lacks GTPase activity and remains active
• See figure 18-11
Ras Pathway
• Growth factor binds receptor
• Receptor exchanges GTP for
GDP on Ras
– Ras activated
• RasRafMekMap
Kinasetranscription
factors genes turned on
Mutant Ras Protein
• Single amino
acid changes
create N-ras
and K-ras
variants
p53 Tumor Suppressor Gene
• Mutated (inactivated) in more than 50% of all
cancers
• p53 regulates (activates or represses)
transcription of more than 50 different genes
• p53 regulated by Mdm2 (prevents the
phosphorylations and acetylations that activate
inactive p53)
• Activated p53 levels rise rapidly if DNA is
damaged or repair intermediates accumulate
P53 Function
• Activated p53 acts as transcription factor to turn
on genes that
– arrest the cell cycle so DNA can be repaired
• Initiates synthesis of p21, which inhibits
CDK4/cyuclinD1 complex, blocking entry into S phase
• Genes expressed which retard rate of DNA replication
• Other products block G2/M progression
– Initiate apoptosis if DNA cannot be readily repaired
• Turns on Bax gene, represses Bcl2 gene
• Bax homodimers activate process of cell destruction
• Cancer cells lacking p53 do not initiate pathway even if
DNA/cellular damage is great
RB1 Tumor Suppressor Gene
• Retinoblastoma 1 gene
• Involved in breast, bone, lung, bladder and
retinal cancers (among others)
• Inheriting one mutated (inactivated) copy of
gene increases chances of retinoblastoma
formation from 1/14,000-20,000 to 85% (plus
increases other cancer rates)
– Loss of second copy in a cell eliminates function
– Normal cells unlikely to lose both good copies
pRB Function
• Tumor suppressor protein that controls the G1/S
checkpoint
• Found in nucleus and activity regulated by level of
phosphorylation (by CDK4/cyclinD1 complex)
– Nonphosphorylated version binds to TFs such as E2F,
inactivating them
– Free E2F and the other regulators turn on >30 genes
required for transition to S phase
Familial Retinoblastoma
Inherited Predisposition for Cancer
• About 1-2% of cancer has an inherited or
familial component
– 50 different forms known at present
• Inherited in Mendelian fashion but most all
genes/alleles are recessive
– Second copy must be mutated in a somatic cell
• Called loss of heterozygosity (and loss of function)
• Loss of second copy in germ line lethal
• RB1 and APC (lost in FAP, familial
adenomatous polyposis) are examples of such
genes
Multistep Development of Colon
Cancer
• APC loss causes cells to partially escape cell cycle
regulation, DCC seems to be involved in cell adhesion
and differentiation
Transforming Viruses
• Viruses discovered to cause cancer in animals
– Acute transforming viruses
• Commonly but not always retroviruses
– Rous sarcoma virus (RSV) discovered by Francis
Peyton Rous discovered in 1910 as a causative agent
of chicken sarcomas (solid tumors of muscle, bone or
fat)
• Many years later shown to be retrovirus
• Nobel Prize in 1966 (link of viruses to cancer)
Retroviruses
• ssRNA chromosome
• Chromosome copied to DNA by reverse transcriptase
upon entry into cell
• DNA integrated into host cell chromosome
– Provirus
• Provirus has strong promoter elements in U5 and U3
terminal sequences
– U5 expresses gag, pol and env
• Oncogenic when
– Integrate near proto-oncogene and cause inappropriate or over
expression
– Bring v-onc as part of viral chromosome
Retroviruses
• Many transforming retroviruses are defective in the
sense that one or more of gal/pol/env have been
deleted to make room for the v-onc
Viral Oncogenes
• Most v-onc genes have normal cellular counterparts
– If simply mutated to the oncogenic form and not in a
virus are called c-onc
Human Cancer-Associated
Viruses
• To date no acute transforming retroviruses have
been discovered in humans
– Viruses can contribute to but not be the sole cause of
human cancer
– However, up to 15% of all cancers have a viral
association
• Papillomaviruses HPV 16 and 18, hepatitis B virus,
Epstein-Barr virus, Human T-cell leukemia virus are
examples of cancer-associated viruses
Human Viruses Associated With
Cancer
• Non-retroviral varieties
• Many of these v-onc genes act to stimulate the cell cycle
(viruses needs host replication apparatus to multiply
V-onc Gene Product Action
• Some v-onc gene products have their
transforming effect by binding and thereby
“taking out” certain tumor suppressor gene
products
– Cell division required to provide replication
apparatus for virus
– Bad, but does open some interesting treatment
possibilities…
Environmental Agents and Cancer
• Natural and man-made carcinogens
– Chemicals, radiation, chronic infections
• 30% of cancer deaths associated with cigarettes
– Seems to preferentially mutate proto-oncogene and tumor
suppressor genes
• Red meat consumption
– How cooked?
• Alcohol-based inflammation of the liver
• Aflatoxin (mold on peanuts)
• UV light or ionizing radiation
– Radon gas (up to 50% of radiation exposure???)
Oncogenes that code for cytoplasmic protein kinases
involved in cell proliferation: src, raf
Ras activates
Raf kinase.
Raf activates
MAPKs.
MAPKs
phosphorylate
and activate
transcription
factors Elk-1,
SRF, AP-1 that
regulate Cyclin
expression
Oncogenes that encode nuclear transcription factors:
AP-1: fos, jun
Translocation of c-myc in Burkitt’s lymphoma
•Translocation of c-myc transcription factor gene to Ig loci
• c-myc normally induced in response to growth factors
•Translocation results in constitutive activation because of being
under control of Ig promoters of the B-cells
Oncogenes that code for cytoplasmic protein kinases
involved in cell proliferation: abl
Chronic myelogenous leukemia (CML) is a clonal
hematopoetic stem cell disorder
The cytogenetic hallmark of all phases of CML is the Philadelphia (Ph)
chromosome. The Ph chromosome is a shortened chromosome 22 that results
from a reciprocal translocation between chromosomes 9 and 22.
Fusion of the c-abl oncogene from chromosome 9 with sequences from
chromosome 22, the breakpoint cluster region (bcr) gives fused bcr-abl gene.
Conversion of abl-protooncogene into an oncogene
in myelogenous leukemia
Depending on the site of the breakpoint in bcr, different fusion proteins are
produced: p185 (185 kDa), p210 (210 kDa), or rarely p230.
How does bcr-Abl fusion protein expression cause
leukemia?
c-abl, the cellular homolog of the transforming
protein found in the Abelson murine leukemia
virus (v-abl), encodes for a nonreceptor tyrosine
kinase.
The c-abl protein has tightly regulated kinase
activity and shuttles between the nucleus and
cytoplasm. Abl is a Src family tyrosine kinase that
has SH2, SH3 domains and can activate MAPK
signaling.
bcr-abl fusion proteins are exclusively cytoplasmic
and have enhanced tyrosine kinase activity that
causes excess proliferation of myeloid cells.
Genetic mechanisms underlying Retinoblastoma
How to lose the remaining good copy of a tumor
suppressor gene
Rb mechanism of action in cell cycle
regulation
Phosphorylation of Rb by Cdk results in activation of E2F
family transcription factors
Cyclins
No functional Rb = no inhibition of E2F activity = unchecked cell division
• Li-Fraumeni syndrome (LFS) is a cancer predisposition syndrome associated
with soft-tissue sarcoma, breast cancer, leukemia, osteosarcoma, melanoma,
and cancer of the colon, pancreas, adrenal cortex, and brain. Individuals with
LFS are at increased risk for developing multiple primary cancers.
• More than 50% of individuals diagnosed clinically have an identifiable diseasecausing mutation in the P53 gene. Of these mutations, 95% can be detected by
sequence analysis,
• Genetic counseling. LFS is inherited in an autosomal dominant manner.
Offspring of an affected individual have a 50% chance of inheriting the diseasecausing mutation.
DNA damage induces p53 expression
P53 mechanism of action in cell cycle
regulation
DNA damage increases
p53 expression of
normal cells.
P53 acts as a
transcription factor to
induce p21 (cip1), a cdki.
Blocks cell cycle
progression.
If the DNA damage
cannot be repaired p53
will induce transcription
of Bax, PUMA, proapoptotic Bcl-2 family
members.
Cell cycle regulation by tumor suppressors
Bax, Noxa,
PUMA
Fas
DR4,5
Replication of damaged DNA can lead to
chromosomal abnormalities, gene
amplification, and gene loss
APC = adaptor, brings GSK-3 close to β-catenin so it can be Phosphorylated and
degraded
No APC = lots free β-catenin = increase in gene transcription = cell proliferation
DNA damage repair by BRCA1
This pathway is dysfunctional in some breast cancers
BRCA, the breast cancer gene, missing in only 5% of breast cancers, BRCA (-) genotype has 60% risk
of breast cancer by age 70.
ATM, ataxia-telangiectasia mutated
Suppression of cell survival by PTEN
Genetic Alterations in Colon
Carcinoma
1. Loss of APC leads to early adenoma
2. Constitutive activation of ras leads to
intermediate adenoma
3. Loss of smads and TGFbR lead to late
adenoma
4. Loss of p53 leads to carcinoma
5. Other alterations can lead to metastasis
Apoptosis
• Programmed cell death, cell suicide
• Pathway should be activated if “something goes
wrong”
– Especially involving DNA/chromosome damage
• Involves proteases called caspases
• Regulated by Bcl2 and BAX
– BAX homodimer promotes apoptosis, Bcl2 homodimer
blocks apoptosis
– Some cancer cells overproduce Bcl2 and are resistant to
some chemotherapies and radiation treatment
• Proteins involved in cell cycle checkpoints regulate
pathway
Control of Apoptosis
Oncogenes encoding products that
affect apoptosis
TRANSFORMATION
Initial event
PROMOTION
Immortalization
Altered DNA
repair
Inhibition of
apoptosis
Successive
accumulation
of mutations
Cell cycle
promotion
Inhibition of
apoptosis
PROGRESSION
Genomic instability
Other mutations
Irregular expression
of apoptosis
Drug resistance
Two Major Pathways of Apoptosis: 1. Receptor-regulated
2. Mitochondrial
From:
“The biochemistry
of apoptosis”
Hengartner, 2000
Nature
407:770-776
Differentiation, development
and programmed cell
death.1999, The molecular
basis of cell cycle and growth
control. Ed. Stein, Baserga
and Denhardt, 305-307, WileyLiss, Inc.
Oncogneic activity of Bcl-2 and Bcl-xL
Structure of Bcl-XL with a
BH3 peptide bound.
Adams and Cory, 1998. Science
281:1322-1326.
APaf-1= apoptosis protease activating factor-1
CARD=caspase recruitment domain
Apoptosome: contains cytochrome C, ATP, caspase9, APaf-1
Oncogenes encoding cell survival signals: akt
Akt activity in blocking apoptosis